Processing lead alloy containing magnesium



United States Patent Int. Cl. C22c 11/02 US. Cl. 75--167 4 Claims ABSTRACT OF THE DISCLOSURE Lead shot consisting of 1-2.5 percent by weight of magnesium, up to about 2.5 percent by weight of rare earth metal, and the balance substantially lead, characterized by the eutectic composition within the alloy being divorced and the precipitated Mg Pb in the alloy being agglomerated into elongated particles connected together in a substantially continuous manner along the grain boundaries of the alloy. Such lead shot falling in water, e.g., rivers, creeks, lakes, ponds, disintegrates or become friaible within a day or two and ceases to be a poisonous hazard to bottom-feeding water fowl. The method of preparing such shot includes the critical step of cooling the molten allow relatively slowly so that solidification of the eutectic composition does not take place in less than about 0.1 second, and preferably takes place during 1-15 seeonds. The alloy can be shotted, cast in pellet molds, or cast as ingot and mechanically reduced to spherical shot.

The invention relates to a novel method of processing lead-base alloy containing magnesium and to lead shot prepared from such lead alloy.

The problem of poisoning ducks and other bottom feeding water fowl with lead shot which has missed the target and fallen into shallow water has been recognized for a long time. The problem has become more serious because the increasing number of hunters has increased the incidence of residual lead shot in popular hunting areas. Several answers to the problem have been proposed and tested but none seems to have been completely successful. For example, it has been proposed to add a small amount of magnesium to the lead in order to make the shot disintegrate during immersion in either lake water or sea water. However, lead alloy containing up to about 5 percent magnesium and shotted in a shotting tower tends to crack but does not readily disintegrate nor lbecome friable. Lead containing larger amounts of magnesium becomes increasingly reactive and disintegrable but the density of the shot is sufiiciently lowered that the shot does not carry well. As a consequence accuracy in shooting is substantially decreased.

It is a principal object of the present invention to provide a method of processing lead-base alloy containing relatively low amounts of magnesium in such a manner as to make the alloy more readily disintegrable during immersion in water.

Another object of the invention is to provide lead shot which becomes readily disintegrable in water.

These and other objects and advantages of the present invention will be more clearly understood by those skilled in the art upon becoming familiar with the following description and the illustrative examples.

The invention is based on the discovery that upon providing lead alloy comprising from about 1 to 2.5 percent by weight of magnesium, up to about 2.5 percent by weight of rare earth metal and the balance substantially lead, and further, on providing the said alloy in molten form and cooling the alloy relatively slowly "ice whereby eutectic solidification takes place in not less than about 0.1 second, the solidified alloy in the form of shot having diameters not greater than about 0.25 inch becomes disintegrable during 1 to 5 days immersion in water. Generally the alloy in shot size disintegrates or becomes readily disintegrable in 1 to 2 days. More preferably the alloy in the form of pellets from about 0.12 inch to about 0.25 inch in diameter is cooled sufiiciently slowly that eutectic solidification takes place in about 1 to 15 seconds. Smaller pellets may be solidified even more rapidly, pellets having an average diameter of about 0.03 inch to about 0.12 inch being cooled so that eutectic solidification takes place, in a preferred manner, in about 0.1 to 1 second. Most billets readily cool at the requisite slow rate required. Preferably the alloy when cast as a billet and then reduced to shot contains at least 0.2 percent by weight of rare earth metal in addition to magnesium, and more preferably at least 0.5 percent by weight of rare earth metal. The method of the present invention is applicable no matter how the lead alloy is transformed into shot, whether by shotting, by casting into pellet molds, by casting as a billet which is then extruded, cut up and the pieces rounded, or by casting as a billet, machining into chips and compressing or agglomerating the chips into shot.

The theoretical basis for the invention is not completely understood. When the alloy is solidified rapidly, the eutectic structure consists of fine particles of Mg Pb well dispersed in the solidified lead alloy. However, when the alloy is solidified slowly, the eutectic composition, as it solidifies, is divorced the Mg Pb precipitate agglomerates to form relatively large elongated particles which tend to be connected together in a substantially continuous network along the grain boundaries of the alloy. These coarser Mg P b phases appear to be much more subject to attack by water and the attack along the grain boundaries facilitates cracking and disintegrating of the lead alloy.

Lead alloy with no more than about 2.5 percent by weight of magnesium has suflicient density for accurate shooting of shot made from the alloy. The alloy may also contain up to about 2.5 percent by weight of rare earth metal. Rare earth metal additions do not decrease the density of the alloy as greatly as magnesium and the rare earth metal exerts some beneficial effect upon the disintegrating qualities of the alloy.

The method of the present invention is particularly effective when practiced upon lead alloys containing at least 2. percent by weight of magnesium when rare earth metal is not present in the alloy, or, at least about 1 percent by weight of rare earth metal if the magnesium content is less than about 2 weight percent.

For the purposes of the description and the appended claims lead-base alloys are to be understood to comprise at least percent by weight of lead. It is also to be understood that the expressions, disintegrable, water disintegrable shot, and, subject to ready disintegration, refer to shot which does more than exhibit surface cracks or split into two or three major pieces, but cracks into a larger number of pieces or becomes friable or both while otherwise retaining integrity of structure, or further, falls completely apart without mechanical agitation.

Lead alloys used in the present invention are made up in a conventional manner, for example, by melting the metals together. In the case wherein rare earth metal is to be alloyed with the lead, an alloy of rare earth metal and magnesium may be used to introduce the desired amount of rare earth metal, if desired. The lead used in making the alloy is preferably substantially free of antimony, bismuth, tin, or other elements which tend to form solid-insoluble phases or intermetallic compounds with magnesium, as well understood in the art. The lead can be made sufficiently free of such elements, if necessary, by adding magnesium to the molten lead and skimming olf the insoluble phases The Pb-Mg alloy is then analyzed and additional magnesium is added, if needed to make 4 the shot to solidify as non-spherical pellets or even more irregular particles.

The shot prepared according to the invention cracks and is readily broken up into a number of finely-divided pieces after from 1 to 5 days immersion in water so that the composition intended. 5 the shot is no longer attractive to bottom feeding water The cooling rate of the molten metal during solidificafowl as a substitute for gravel for the digestive system tion of the eutectic composition, i.e., Pb-Mg eutectic, is of the bird. critical and must be controlled so that the eutectic compo- The following examples serve to illustrate the invensition does not solidify in less than about 0.1 second. Th tion and are not to be considered as limiting the scope cooling rate during solidification of the eutectic composithereof. tion can be slowed down and controlled by casting the metal while the melt is relatively hot and/or while the FIRST SERIES OF RUNS mold is relatively warm. Since larger masses tend to cool I a series f runs made to illustrate the method and Slowly y y, external heating or controls are more shot of the invention as well as comparison methods and in Casting Smaller P Shot, than in castshot, lead-base alloys were made up which contained ing billets. The melt should be at a temperature of about f 1 to 2 percent by Weight of magnesium, up to 2 600 to 800 F. at the time of casting while the mold prefpercent b weight f misch metal and the balance efahly is heated to a temperature in the range of 400 to mercially pure lead. Misch metal is a commercially avail- 600 F. and more preferably is maintained at a temper- 2O able mixture of rare earth metals containing predomiatllfe of 450 to dlll'hlg solidification 0f the ynantly cerium and lanthanum. The alloys were cast in a To assure complete solidification of the alloy the te ra hite mold having spherical cavities i Order to bperature of the mold must be allowed to fall below about i Py i diameter pellets. casting was carried out While higher temperatures than hecessay are under various combinations of conditions in which the efally 11561658 and to be avoided, melt temperatures of the metal was poured hot or relatively cool and/or the mold order of 800 to 1200 F. may be used in alloys conwas hot or relatively C001. taihihg rare earth metals to avoid Segregation of lead The eutectic composition cooled during an elapsed rare earth metal compound. time less than 0 1 second in the cases where the alloy was The metal may be cast directly into shot-size pellets poured cool and into a relatively cool mold. Therefore not exceeding about 0.25 inch in diameter, and usually p llets cast in this Way were made by a method outside less than about 0.15 inch in diameter. The metal may also the scope of the invention and the results thereof are prebe cast as a billet which is then reduced to rod or wire sented by way of comparison. by extrusion. The rod or wire is cut up and rounded by Pellets obtained under these various conditions were mechanical methods to provide substantially round shot. tested to determine disintegrability in water. Three pellets The cast billets may also be transformed into shot upon from each run were placed together in a bottle half filled machining the billets into chips and and squeezing or with Water. The bottles were set aside and the pellets compressing the chips into substantially spherical shape Were examined visually periodically. When the pellets as in a die using a concave punch and die. cracked the bottle was shaken to determine whether the The disintegration rate of the alloy apparently is still pellets readily would substantially completely disintegrate. further increased by cold working. Therefore, cold re- If all three pellets did not at least crack into a plurality shaping of cast pellets or cold extruding of billet into of pieces, the number which did break up was recorded. rod or wire and subsequent preparation of shot from the The compositions prepared, the casting conditions and the extruded material may be beneficial for certain appliresults of the tests on exposure to water are summarized cations. in the following Table I.

TABLE I Composition Days Until Pellets Days Until Pellets Cracked Disintegrated Run Percent Percent 0. Mg MM H-H C-H H-o C-O H-H C-H H-C cc Conditions of casting are indicated by the symbols H-H, C- H-C, and (3-0.

HH=Hot melt temperature (ea. 1,200 F.), hot mold (ca. 500 CH=Cold melt temperature (ea. 625 F.), hot mold (ea. 500

HC=Hot melt temperature (ca. 1,200 F.), cold mold (ca. 100 F.)

CC= Cold melt temperature (ca. 625 F.), cold mold (ca. 100 F.), comparison method.

Parenthetical values indicate number of pellets cracking or disintegrating where all three pellets did not respond.

1 Composition, by weight, balance lead.

If desired, the metal may also be prepared by shotting in a shot tower having provisions for heating the tower atmosphere so as to reduce the cooling rate. Temperatures approaching 475 F. may be used. The molten lead alloy during shotting is best held at a temperature just above the liquids temperature of the alloy. In general, it will be found necessary to control the heated tower atmosphere by removing oxygen and water vapor therefrom to avoid undue oxidative attack upon the droplets of molten metal, thereby to avoid the tendency of SECOND SERIES OF RUNS In an additional series of runs, a lead-base alloy containing 2 percent by weight of magnesium and the balance lead was cast into inch diameter pellets at various mold temperatures, respectively. In each case cooling curves were recorded and the length of the thermal arrest during freezing at the eutectic temperature was determined. In each case, three of the so-prepared pellets were subjected to testing by immersion in water in the same manner as in the foregoing tests. The results of these tests,

3. The lead shot as in claim 1 which contains from about 1 to 2 percent by weight of rare earth metal.

TABLE II Composition, Casting Mold Thermal temp., arrest percent by Temp., weight Mg F.

F. temp.,F. Time Days Until Pellets Disintegrated 2 700 300 3 days, slightly cracked. 2 700 450 475 1.0 3 days, broke in small pieces. 2 700 500 480 12 3 days, cracked into a number of pieces. 2 700 600 475 11 Do. 2 600 400 2 days, cracked didnt break up. 2 600 500 477 11 6 2 days, cracked and broke up. 2 600 600 475 10. 5 D0. 2 800 400 2 days, slightly cracked. 2 800 500 475 14. 5 2 days, cracked and broke up. 2 800 600 475 10 D0.

1 Balance lead. 2 Not recorded.

The foregoing tests show that pellets cast under conditions under which the pellets freeze rather slowly show a marked difference in behavior on immersion in water in contrast to pellets cast under conditions under which freezing takes place rapidly.

We claim:

1. Water disintegrable lead shot prepared from lead alloy consisting essentially of from about 1 to 2.5 percent by Weight of magnesium, up to 2.5 percent by Weight of rare earth metal, and the balance substantially lead, the lead alloy being characterized by the eutectic composition therein being divorced and containing precipitated Mg Pb, and by a substantial amount of Mg Pb being agglomerated into elongated particles connected together in a substantially continuous network along the grain boundaries of the allow whereby the lead shot readily disintegrates on immersion in water at ambient room temperature and during a period of about 1 to 5 days.

2. The lead shot as in claim 1 in which the alloy contains from about 2 to 2.5 percent by weight of magnesium.

4. The lead shot as in claim 1 which contains from 20 about 1 to 2.5 percent by weight of magnesium and at least 1 percent by weight of rare earth metal.

References Cited UNITED STATES PATENTS OTHER REFERENCES Hansen: Constitution of Binary Alloys, 2nd ed., 1958, McGraw-Hill Co., New York, pp. 911-913.

35 CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 29-122; l4832 

